This invention relates to a servo steering apparatus for motor vehicles having a driving shaft operated by a hand steering wheel and a driven shaft to which an auxiliary torque is imparted through two friction clutches, each engaging in only one rotational direction and both driven by an auxiliary driving source.
Steering reinforcing systems with two oppositely rotating multiple disc clutches are known as disclosed for example in German Pat. No. OS 1630301. When the steering wheel is turned in either direction in such systems, an axial force is exerted upon one of the clutches assigned to each direction of turning, by means of worm gearing between the driving and driven sections of the steering shaft. Such axial force causes frictional contact between a rotating clutch carrier and the driven section of the steering shaft to deliver servo reinforcement power to the steering shaft depending on the manual force imparted to the steering wheel. Steering reinforcing systems of the foregoing type have the disadvantage that both the worm gearing as well as the other driving connections contribute to a high frictional resistance to steering. Thus, the danger exists that because of heavy thrusts generated by road bed loading, one of the clutches will remain engaged to impart servo steering power through the steering gear without any driver initiated steering movement.
Other servo steering apparatus for motor vehicles are known as disclosed for example in German Pat. Nos. OS 2238125 and DR OS 2533940, wherein an electric motor is switched on in response to rotation of the steering wheel, directionally sensed by a transmitter element, so that the motor transmits its torque to the steering shaft either through one clutch for both directions of rotation or through two clutches for each direction of rotation. The foregoing type of steering apparatus has the disadvantage that the armature of the electric motor and the gear and coupling parts associated therewith must be accelerated from standstill in response to every steering movement and in the event of a rapid change of direction, braking of the armature and then acceleration in the opposite direction must be effected. As a result, the steering response is impaired. In German Pat. No. OS 2603306, an auxiliary power steering system for motor vehicles has been described wherein an auxiliary torque is produced through two asymmetrically-built loop or expanding spring clutches. However, these clutches do not have a proportional response characteristic. Moreover, the auxiliary torque persists too long because of the loop springs, so that an over-travel condition occurs. The reduction drive necessary for reducing the speed of the auxiliary driving source, is also a disadvantage of the foregoing type of servo steering system. Because of automatic locking, the auxiliary driving source may not be switched off. Further, this type of system defeats any attempt to achieve a compact and short overall construction.
It is therefore an important object of the present invention to provide a servo steering apparatus which is constructed in a simple and compact manner and which exhibits good proportional response performance.